Typically, for alternating current (“AC”) powered lights, the use of LEDs requires a current limiting device to regulate the current through the LEDs and maintain a constant light output. FIG. 1 is a schematic of a conventional AC LED circuit. The LEDs can be driven directly by the AC power source, for example, a 90 VAC to 135 VAC or 207 VAC to 253 VAC source. In this example, LED strings D1 and D2 are connected in parallel with reverse connections so that only one of the LEDs is biased on during each half cycle. Although schematically only one LED is shown, it is known that the LEDs may he a string or array of LEDs.
The operation of the circuit of FIG. 1 is shown in FIG. 2. Source voltage 202 is a sinusoidal wave form having a positive half cycle and a negative half cycle. During the positive half cycle, LED string D2 is reverse biased and when source voltage 202 exceeds the forward voltage of LED string D1, LED string D1 conducts and emits light during the time shows in area 204. During the negative half cycle of scarce voltage 202, LED string D1 is reverse biased and when source voltage 202 exceeds the forward voltage of LED String D2, LED string D2 conducts and emits light during the time shown in area 206. In this example, the forward voltages of LED strings D1 and D2 are equal and resistors R1 and R2 limit the current through the LED strings D1 and D2 linearly. However, the sinusoidal AC line voltage turns off the LED strings at a rate of double the line frequency. As can be seen in FIG. 2, the LED string on-time is about 40%.
FIG. 3 shows the schematic of an LED string powered by a rectified AC voltage with a linear current source regulating the LED current. AC power source 302 provides a sinusoidal input to bridge rectifier 304. The rectified voltage is then applied to power current source 306 and to forward bias LED string 308. LED string 308 is forward biased only when the output of bridge rectifier 304 exceeds the sum of the forward voltages of LED string 308. FIG. 4 shows a graphical depiction of rectified voltage 402 and the on time for LED string 308 shown by the high signals of curve 404. Similar to the circuit of FIG. 1, this circuit turns off the LED string at a rate of double the line frequency. The LED string on-time is about 40 to 50%.
To retain the simple design of linear mode power supplies for LED arrays while improving the efficiency and reducing light off-time, manufacturers have come up with the idea of powering only part of the LED string when the source voltage is not high enough to turn on the whole LED string. FIG. 5 shows an example of a linear LED driver to drive three LED strings. AC power 502 applies AC power, which is rectified by bridge rectifier circuit 504. Integrated circuit 506, in this example the commercially available integrated circuit AIC6600, contains a voltage detector that is used to control the turning on and turning off of the LED strings based on the detected voltage level of input voltage. At lower input voltage levels, integrated circuit 506 activates main LED string 508 and does not activate LED strings 510 and 512. At higher voltage levels, integrated circuit 506 turns on main LED string 508 and LED strings 510 and 512. This is shown in the graph of FIG. 6 which shows rectified voltage 602 compared with the on-time for the LED strings. During the initial rise in rectified voltage 602, when the voltage level does not exceed the forward voltage of main LED string 508, none of the LED strings are active as indicated by segment 604. When rectified voltage 602 exceeds the forward voltage of main LED string 508, main LED string 508 is powered as indicated by segment 606, with LED strings 510 and 512 being off. When rectified voltage 602 exceeds a second threshold level, main string 508 and LED string 510 are active as indicated by segment 608. When rectified voltage 602 exceeds a third threshold level, main LED string 508, LED string 510 and USD string 512 are active as indicated by segment 610. The number of LEDs in the LED strings is dependent on the voltage level of the AC power source. It can be seen that the LED on-time is increased to 85% with the circuit in FIG. 5. However the usage of LED strings is not even, LED Main String 508 is turned on more often than LED strings 510 and 512, which may cause a difference in the lifetimes of the LED strings.
Therefore, it is with respect to these considerations and others that the present invention has been made.